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"The Towel" R/C Stunt Plane 



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The Towel" R/C Stunt Plane 



Written By: Breck Baldwin 



>f TOOLS: 

Drill with assorted small bits (1) 

Felt-tip pen (1) 

Hacksaw (1) 

Heat gun (1) 

for heat-shrink tubing 

Hex key (1) 

/'/ needed to remove your motor's mount 

Needlenose pliers with wire cutters (1) 

Razor knife (1) 

Ruler (1) 

Screwdriver (1) 

Soldering iron and solder (1) 

if your connectors need soldering 



PARTS: 

Extruded polystyrene insulation board 

(11 

We use Dow High Performance 
Underlay ment-PP (faced with plastic 
film). It weighs just 4oz for the airframe 
and stabilizers. Ask for leftovers at 
construction sites. You can also use stiff 
cardboard, presentation board. 
Coroplast. or Depron foam, but heavier 
materials may require a stronger motor 
and more nose weight. Softer materials 
may cause flutter. 

Corrugated plastic (1) 

such as Coroplast. You can get it from 

recycled signs. 

Servomotors (2) 

with servo extensions if needed. We like 
the HexTronik HXT900 and Turnigy 
TG9e servos. 

Electronic speed controller (ESC) (1) 

Motor (1) 

capable of 15oz thrust using 2 cells 

(7.4V) and a 10x4.7 or 9x6 prop, such 



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"The Towel" R/C Stunt Plane 



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as the classic Tower Pro Outrunner 
2408-21 

R/C transmitter and receiver (1) 
such as Hobby King HK-T6Av2. Check 
hobbyking.com for the latest. It's a 
dynamic market. 

Propellers (3) 

Get a 6- pack; you'll break some. You 
can also use the APC slow-fly 10x4. 7. 
but its hole is bigger and will require a 
different attachment to the motor. 

• Taped) 

• Cable ties (30) 
aka zip ties 

• Aluminum angle (1) 
or similar stock, to make the motor 
mount 

• Connectors (3 pair) 
We used 3.5 mm gold bullet connectors. 

• Heat-shrink tubing (1) 
for soldered connections 

• Connectors (1) 
Match your battery's connector, or get a 
male/female pair. We used 30 A 
Anderson Powerpole here. Also good: 
HexTronik XT60. 

• LiPo battery pack (1) 
capable of 15 amps power draw 

• Propeller saver (1) 
or other size to match your motor and 
prop. If you use the larger APC props 
you might not be able to use a prop 
saver, but those are stronger props 
anyway. 

Coat hangers (2) 

• Taped) 
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"The Towel" R/C Stunt Plane 



aka velcro tape 

Taped) 

the uglier the better 

V-tail mixer or elevon mixer (1) 

if your transmitter doesn't have elevon 

mixing 



SUMMARY 

The great power of the Towel is that everyone thinks they can make one — and they're right. 
Stupid-simple to build, all it takes is a spare afternoon, $100 worth of gear, and some DIY 
chutzpah. 

The Towel is a great-flying airplane that's optimized for typical urban flying conditions: gusty 
winds, small flying spaces, and rough landing spots. Unlike store-bought beginner planes, 
the Towel has a 1:1 thrust-to-weight ratio that makes it highly maneuverable. This allows it 
to fly in tight spaces and turbulence. It can also carry a camera. 

Lots of people have learned to fly on the Towel. Repairs are simple and the airframe can 
take a lot of punishment before needing replacement, which takes minutes. It's made from 
recycled materials and designed to not seriously hurt people or property. 

Why the Towel moniker? Back in the day, I was flying an early version of the plane that had 
met Mother Earth at aggressive velocities many times. The nose had become a rumpled 
shadow of its former self. A fellow pilot, who was a bit of a smartass, remarked that it looked 
like I was trying to launch a wet towel, and the name stuck. 

The Towel's detachable deck is an innovation in DIY hobby flying. You'll spend 80% of your 
build time on the deck, and only 20% on the airframe and control surfaces. This allows for a 
very desirable property of the Towel, which is that the airframe can be easily replaced in that 
20% time frame. We can all thank Mark Harder (aka Splinter) for the deck concept. 

We estimate that well over 100 Towels have been built, by kids and by vastly older kids. 
Here's how you can make one. 



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"The Towel" R/C Stunt Plane 



Step 1 — Practice flying. 



r fFMSl Simulateur de Vol de Modeles-reduits 



■ 



-inh 



Fichier Modele Paysage Vue Simulation Audio Resolution Affichage Controles Language Aide 

Vil: 14 m/s 
Alt: 1 m 
Vw: -0.3 m/s 




• While you wait for your parts to 
arrive, get a flight simulator. Free 
ones include FMS 
( http://n.ethz.ch/~mmoeller/fms ) for 
Windows, and CRRCSim for most 
platforms 

( http://sourceforge.net/apps/mediawiki/cr. .. ). 
Program your game controller to 

work like an R/C transmitter: up 
stick is down pitch. Many 
transmitters also have USB 
interfaces that can drive the flight 
simulator as well. 

• Practice flying directly at 
your head, to get the 
left/right control reversal sorted 
out. You'll learn a lot faster if you 
get most of your crashing done 
virtually. 

• The Towel is constantly being 
improved; check 

http://brooklynaerodrome.com for 
the latest instructions with 
supporting videos, parts lists, and 
tips on sourcing. 



it 



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"The Towel" R/C Stunt Plane 



Step 2 — Charge your flight battery. 




• The dirty little secret of R/C cars/planes/boats is that you'll spend more money on battery 
chargers than most any other single item. Check http://brooklynaerodrome.com to see 
what chargers are working well and are a good value. 

• All modern LiPo batteries come with a charging connector that's separate from the power 
leads. Pictured here is a range of chargers that cost from $8 to $100. Most chargers 
require 12V DC power, which can come from a car battery or a dedicated AC-adapter 
power supply as seen at the top left. 

• CAUTION: LiPo batteries can start fires if they're overcharged, over-discharged, or 
physically damaged. Never charge them unattended or leave them in a flammable 
environment after a crash. Charging them on ceramic tile or in a flowerpot is a good safety 
move. 



A 



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"The Towel" R/C Stunt Plane 



Step 3 — Build the deck. 





• Draw out the deck on your Coroplast following the deck template, and use a ruler and box 
cutter to cut it out. Coroplast is a difficult material to work with, and multiple light passes 
with a very sharp knife work best. Muscling it just annoys the Coroplast and generally 
results in a sliced finger. The Towel is not meant to be a Band-Aid. Keep your blood off it 

• Save the scraps, you'll need them later. 

• It's critical that the servos be well and solidly attached to the deck, and that the 
elevons be well connected to them. These are typical weak spots on deck builds; if 
they're not well attached, then on every landing or crash, the servos shift and the trim of 
the Towel is lost. 

• Out-of-trim airplanes are very hard to fly because they require constant control input for 
level flight. 

• The length of the control rods is the only thing on the deck that must be measured 
precisely within ±Vz of precision. 



* 



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"The Towel" R/C Stunt Plane 



Step 4 — Mount the servos. 




• Peel the labels off your servomotors and clean them with alcohol or another solvent if 
they're greasy. 

# Apply double-stick tape to the servos and attach them to the deck, with their wires toward 
the front and their shafts toward the outside edges. If the wires are short, you may need to 
move the servo closer to the propeller hole, or right to its edge. The prop hole is designed 
around the shortest servo leads we've found. 



Step 5 




• The tape keeps the servo in place but it needs reinforcement from zip ties. 

• Place the holes for the zip ties very close to the servo housing so the servo can't move. If 
the ties are too short they can be doubled easily, as shown here. 

• Crossing the zip ties yields a strong bond to the deck to withstand crashes. 







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"The Towel" R/C Stunt Plane 



Step 6 — Make the motor mount. 




• Use a hacksaw to cut the angle stock to the shape shown, as wide as you need it. For this 
build, I needed 1". 

• For our build we use the motor's 3 existing holes as guides to drill through the mount, then 
attach with zip ties. This motor required a bit of clearance in the center, so a fourth hole 
was needed. 

• Outrunner motors have a mounting setup that you'll have to MacGyver into 
connection with the airframe. Motor mounts can be made using anything from steel 

to 3D-printed plastic . Aluminum angle stock usually works (my favorite source is the steps 
from a worn-out ladder.) 



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"The Towel" R/C Stunt Plane 



Step 7 




• Next, drill holes in the motor mount for attaching it to the deck. 

• Attach the prop saver to the motor and make sure the screws are tight without stripping 
the prop saver's aluminum threads. Usually the propeller shaft is cut flush with the end of 
the prop saver. 

• The holes closest to the motor are offset. This reduces a common stress riser 
(bending/breaking point) that occurs during crashes if the holes are aligned. On the 
front side it doesn't matter. 



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"The Towel" R/C Stunt Plane 



Step 8 — Connect motor to ESC. 






• Connect the speed controller to the motor and to the battery. The male connectors go on 
the motor, female on the speed controller. 

• Strip and tin all wires, 3 from the motor and 3 from the speed controller. Why 3? The speed 
controller runs the motor by applying current to 2 of the 3 leads at a time, in AC, believe it 
or not. There is a genie figuring it all out inside the speed controller. I have seen it escape 
in a puff of smoke on many occasions. 

• Our default setup, which is the "not get laughed off the model plane field" setup, 
requires soldering fussy little 3.5mm bullet connectors for the motor and ESC, and 
insulating the connections with heat-shrink tubing. 

• This is an opportunity for you to innovate and do something different. (We've flown 
planes where the connections were twisted together by hand and wrapped in electrical 
tape.) 

• The trick to soldering the bullet connectors is to stick the end of the soldering iron into a 
small hole in the connector, fill the end with solder, and dunk the tinned lead into the 
bucket of molten solder for a bit until everything is all nice and melted. Let it cool. If your 
fingers are getting burnt, you're doing it right. 

• Heat-shrink it all up. 



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"The Towel" R/C Stunt Plane 



Step 9 — Connect battery to ESC. 




? 



• You have a broad choice of battery 
connections for the speed 
controller; we like Anderson 
Powerpoles. Most batteries come 
with some sort of connector, so 
you can just adopt the matching 
connector in that case. 

• Shown in the photo at top is 
the standard Deans 
connection and at bottom the XT60, 
both of which require soldering and 
can be difficult to separate. In the 
middle is our preferred setup, with 
Anderson Powerpole connectors. 

• CAUTION: Be very careful 
to get polarity correct. 
Reversed battery connections will 
destroy both the battery and the 
speed controller. 



A 



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"The Towel" R/C Stunt Plane 



Step 10 — Mount the prop. 




• Attach the prop to the prop saver, with its raised lettering facing forward. If your prop has 
no lettering, look closely at its blades to see the airfoil and intuit the correct orientation. Or 
do what I do — run it both ways and see which generates more thrust. 

• Attach the motor to the motor mount using zip ties. Center the prop in the middle of the 
deck's prop hole, keeping the prop blades about %" away from the back edge. Punch holes 
in the deck through the motor mount's attachment holes and secure with zip ties. 



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"The Towel" R/C Stunt Plane 



Step 11 — Connect the radio receiver. 




• Follow your receiver's instructions 
to connect it to the right servo 
(channel 1), left servo (channel 2), 
and ESC (channel 3). If you have 
an onboard elevon mixer (not 
shown), install it according to its 
instructions. 

• Turn the transmitter on with the 
throttle (left stick) and its trim tab 
both down. Connect the battery to 
the ESC, and the plane should 
come to life: the servos move with 
the right stick, and the motor runs 
with the left stick. 

• Secure all components to the deck 
with zip ties to protect against 
crashes, which will be a forward 
force, and to be sure that nothing 
can get pulled backward into the 
prop (like the wires shown here). 



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"The Towel" R/C Stunt Plane 



Step 12 — Align the servo arms. 






• Drill out the servo arm to the diameter of a coat hanger. 

• Center the 2 trim tabs on the transmitter's right stick, for elevator (up/down) and aileron 
(left/right). Turn on the transmitter and power up the deck. 

• Attach both servo arms as close to 90° as possible and add their retaining screws. If 
necessary, use the aileron trim tab to make their angles match exactly. Retain this trim 
when bending and mounting the elevon control rods. 

• Some radio manufacturers (such as JR) have a different mapping for the channels. 
Consult your manual for elevon setup. 

# Make sure the motor is turning clockwise, looking from back to front; if not, switch any 



2 of the 3 bullet connectors to reverse its rotation. 

If you're a beginning R/C pilot, drill the servo arm closer to the axis of rotation. This 
will shorten the control throws and make the Towel easier to fly. In Step 10c you 
can see I've drilled for both beginner and advanced settings. 



* 



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"The Towel" R/C Stunt Plane 



Step 13 — Bend the control rods. 




# Cut the straight bottom portion out of a coat hanger and bend a U-shaped hook on one end 
as shown. It's critical that the hook be just wide enough to accommodate the servo arm — 
if the bend is too wide, the controls may slip out of trim or bind on the servo. (Good fit is 
highlighted blue, bad fit is highlighted red.) 



Step 14 — Make the airframe. 




• Follow the template to cut the airframe out of the polystyrene foam sheet. 

• Cut the elevon free by making a single beveled cut of around 30° as shown here. The 
degree of bevel isn't crucial, but try to keep it constant for the length of the elevon. 

• Use a sharp blade, because ragged edges will compromise all your hard work for 
tight servos. 



* 



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"The Towel" R/C Stunt Plane 



Step 15 




• Flip the airframe over facedown 
while leaving the elevon in place 
face up. Fit them together and 
accurately tape in a few places the 
joint that's formed by the beveled 
cut. Then apply a strip of packing 
tape the length of the elevon. Don't 
sweat the wrinkles. Only this side 
needs to be taped. 

• Make a Vi" slot in the elevon mid- 
span, to create 2 elevons that 
move independently. 

• Flip the airframe face up again so 
the hinge is on the work surface. 
Notice how the bevel works to 
allow the elevon's upward motion. 



Step 16 — Attach the deck. 




• Align the prop holes in the deck 
and the airframe, and attach them 
with 6 zip ties that pass through 
both pieces. 

• CAUTION: Keep the 
airplane powered up until 
the elevon control horns are 
mounted, to ensure that trim is 
maintained. Be careful to not hit the 
throttle by mistake. 



a 



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"The Towel" R/C Stunt Plane 



Step 17 — Mount the rods. 






Attach the control rods to the servos, with the rods running outside the servo arms. Use 
the pliers to make a 90° bend in each rod, just aft of the hinge, bending toward the center 
of the airframe. 



Step 18 




• Take V/2" of the prop cutout material from the deck and cut it in half. Force the end of the 
control rods through the flutes of the Coroplast, crosswise, to make 2 control horns. (You 
can't use a flute as the hinge point because it's too big and sloppy.) Lay the airframe flat 
and attach each control horn to its elevon with 2 zip ties. 

• It's important to align the bend points for both rods to ensure that the elevons move 
the same distance. If one is too long, you can use the trimming technique 
discussed in Step 16 to match them up — otherwise get another hanger and try again. 

• Start making airplane sounds; you're almost done. 



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"The Towel" R/C Stunt Plane 



Step 19 — Make the stabilizers. 




• Cut out the 2 stabilizers following the template, after first reinforcing the lugs with packing 
tape on both sides. Cut with extra care around the lugs. 



Step 20 




• Line up the stabilizers right next to the control rods about W forward of the hinge. 
Carefully cut the slots for the stabilizer lugs. 

# Slightly too tight is better than too loose, other- wise the stabilizers will wobble. 
(This can be fixed with tape easily enough.) 

• If built perfectly, the elevons should lie flat on the table. Generally that's not the case, 
so don't worry. 







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Have Towel, Will Travel 



Step 21 — Adjust the trim. 




• Put the airframe on a flat surface, power up the plane and transmitter, and recheck that the 
servo arms are aligned, with any aileron trim tab adjustments. The goal is Vi" of "up trim" 
in both elevons. This is the thickness of the blue foam. If you have more than V4" up trim 
already built in, then the control rod will have to be remade. 

• If there's less than Vi" up trim, then bend a "dogleg" in the control rod with the needlenose 
pliers while gripping it firmly, to effectively shorten the control rod. 



Step 22 — Mount the battery. 






• Stick the hook side of the velcro tape to the Towel, half on the deck and half on the 
airframe. Apply the loop side of the velcro to the battery pack. Make sure the battery is 
charged, then attach it to the Towel. 

• Test the center of gravity (CG) by balancing the plane on your fingers. The Towel works 
really well with its CG at 10" from the nose. Adjust the CG by moving the battery pack's 
position. That's it! 



Have Towel, Will Travel 

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Have Towel, Will Travel 

Know Your Controls 

Throttle: The transmitter's left stick is the throttle: push it forward to go faster, pull it backward 
to go slower. 

The transmitter's right stick operates the elevons as follows: 

Pitch: Elevator signal determines the plane's up/down attitude, or pitch. Pull the stick back to angle the nose up (both 
elevons go up), and push it forward to nose down (both elevons go down). It's the reverse of a game controller. Roll: Aileron 
signal determines the plane's roll to the left or right. (It's not really left and right turning — that's yaw, and the Towel doesn't 
use the rudder signal.) Push the stick left to roll the plane left (raise the left elevon and lower the right elevon). Push it right 
to roll right (raise the right and lower the left elevon). 

Elevon mixing: If your radio supports elevon mixing, follow the manual to achieve it. An onboard 
elevon mixer, aka V-tail mixer, should also provide instructions; it's shown here (inset) 
connected but not yet zip-tied in place. 

Control throws: The beginner-level control throws are ±W in both pitch and roll. If you know 
how to fly, then double that. Adjust the throws by either using dual rates on your transmitter or 
moving the servo horn holes inward (reduce throw) or outward (increase throw). 

Flight School 

A well-built Towel takes damage like a B-17 bomber takes flak — if the motor's still turning, then 
it's flying. A few tips for successful flight: 

CG and trim: The plane built for this article flew perfectly with the trim and center of gravity 
shown here. Get this right - it's critical. 

Launching: I launch the plane with my left hand, while my right thumb keeps constant contact 
with the right stick. I run up the throttle with my chin, up to about two-thirds throttle, and release 
smoothly into the air. I do not throw the Towel, nor do I drop it. It's an easy, straight toss. Since 
my thumb is on the elevon controls, it's easy to correct wind disturbances and "off" launches. 

Flight training: If you're a newbie, we recommend the Splinter Method of flight training. This is 
best done over deep grass in a big field with nothing to hit. 

Have someone else launch the plane with 50% power. The pilot's job is to keep the wings and nose level and cut throttle 
before the plane settles into the ground, which it should do in a few feet at 50%. Once that's working well, increase power to 
75%. Fly the plane straight for 50 feet and land it by cutting throttle. Repeat until this is smooth. The pilot then launches their 
own plane at 75% power for a 50-foot flight. No turns !Once comfortable with launching and landing, the pilot will attempt to 
turn 180° and then land. Any sign of trouble and the immediate reaction is to cut throttle and keep the nose level. You 
should note that when the Towel flies at you, the left and right controls are reversed! Flight simulator time will really help with 
this. The pilot will then attempt a 360° turn and landing. Now the pilot can attempt figure-eights and further skill building. 

Flight time: Standard battery packs in the range suggested will last 6-7 minutes, and take about 
25 minutes to charge. Not all speed controllers have reliable automatic cutoffs for low voltage, 



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Have Towel, Will Travel 
so be mindful. 

With that said — fly your Towel, send us a picture at http://brooklynaerodrome.com . make 
something different, improve upon the design, and share your build at makeprojects.com . 

This project first appeared in MAKE Volume 30, page 82. 

This document was last generated on 201 2-1 0-30 07:1 2:59 PM. 



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